The present invention relates to the art of protection circuitry for a power amplifier and, more particularly, for protection circuitry for an RF power amplifier of the type employed in AM radio broadcasting.
It is known in the art to provide VSWR protection circuitry for use in protecting an RF power amplifier to insure that the power amplifier is not damaged at high levels of VSWR. Such protection circuits, for example, will sense the forward power level and the reflected power level and control the transmitted output power when the ratio of the reflected power to the forward power becomes too great. Such a circuit is disclosed in U.S. Pat. No. 4,353,037 to G. D. Miller.
Another example of an RF power amplifier having VSWR protection circuitry is presented in my U.S. Pat. No. 4,859,967 entitled "RF Power Amplifier Protection", which issued on Aug. 22, 1989, the disclosure of which is incorporated herein by reference.
As discussed in my aforesaid U.S. Pat. No. 4,859,967 such RF power amplifiers frequently include transistors which may be damaged if operated at high levels of VSWR. These transistors may take the form of MOSFET transistors connected together in a bridge circuit. Failure of these transistors due to high VSWR conditions may be prevented by turning off the power amplifier for a short period of time after detecting a high VSWR condition. Most VSWR protection circuits will turn an amplifier off at a certain level of VSWR even though the amplifier may not be under stress. That is, most amplifiers can tolerate a wide range of impedance variation so long as the variations present a resistive load to the power amplifier. Turning off an RF power amplifier at a certain level of VSWR may be unnecessary as the amplifier may not be under stress during a resistive load. This can lead to needless interruptions of RF transmitter service.